The phrase once in a blue moon is used for for rare events. The second full moon in a month is called a blue moon. It occurs every two or three years, always on the 30th or 31st of a month. Sometimes, like in 2018, it is twice in a year, then usually in January and March, or in very rare cases in January and April, like in 1961. The span between two full moons is about 29.5 days and only February has less than that, so there is never a blue moon in February. You will find each moon phase in the lunar calendar.
April 30, 1942 October 31, 1944 August 31, 1947 May 31, 1950 December 31, 1952 October 31, 1955 July 30, 1958 January 31, 1961 April 30, 1961 November 30, 1963 August 31, 1966 May 31, 1969 December 31, 1971 October 31, 1974 July 30, 1977 March 31, 1980 December 30, 1982 July 31, 1985 May 31, 1988 December 31, 1990 September 30, 1993 July 30, 1996 January 31, 1999 March 31, 1999 |
November 30, 2001 July 31, 2004 June 30, 2007 December 31, 2009 August 31, 2012 July 31, 2015 January 31, 2018 March 31, 2018 October 31, 2020 August 31, 2023 May 31, 2026 December 31, 2028 September 30, 2031 July 31, 2034 January 31, 2037 March 31, 2037 October 31, 2039 August 31, 2042 May 30, 2045 January 31, 2048 September 30, 2050 July 30, 2053 March 31, 2056 October 31, 2058 |
The original meaning of blue moon was different. A normal season (spring, summer, autumn, winter) had three full moons, called first, second and last. When there were four full moons in a season, the third one was called blue moon.